Any of a wide variety of exercises, activities or devices used in the language classroom for realizing lesson objectives is known as—

The correct answer is technique.

In language teaching methodology, there is a clear distinction between an approach, a method, a syllabus, and a technique (as conceptualized by Edward Anthony and modified by Jack C. Richards and Theodore Rodgers):

1. Technique: A technique is a specific, concrete exercise, activity, or device used inside the classroom to accomplish an immediate lesson objective (e.g., flashcards, role-playing, drills, or a matching game).

2. Method: A method is an overall plan for the orderly presentation of language material based upon a selected approach.

3. Approach: An approach is a set of correlative assumptions dealing with the nature of language learning and teaching (the philosophical basis).

4. Syllabus: A syllabus is a document that outlines the goals, content, and sequence of topics to be covered in a course.

Therefore, any specific activity or device used directly in the classroom for realizing lesson objectives is classified as a technique.